If someone wants to set up Closure compiler and leave the rule commented out like we did for print preview, I can then help fix it up.

Thanks! I'll setup the initial compilation target shortly.

Initial dummy setup is at https://codereview.chromium.org/2879543002.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/367dcbb346040d38334453592c0f61d827a00cae

commit 367dcbb346040d38334453592c0f61d827a00cae
Author: dpapad <dpapad@chromium.org>
Date: Wed May 17 03:42:53 2017

PDF Plugin: Add dummy JS compilation target.

- Uncomment line at third_party/closure_compiler/compiled_resources2.gyp:36
- Execute ./third_party/closure_compiler/run_compiler main

The dummy compilation target will facilitate the work needed to
fully type check the PDF Plugin's JS codebase.

BUG=721073
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2879543002
Cr-Commit-Position: refs/heads/master@{#472309}

[add] https://crrev.com/367dcbb346040d38334453592c0f61d827a00cae/chrome/browser/resources/pdf/compiled_resources2.gyp
[modify] https://crrev.com/367dcbb346040d38334453592c0f61d827a00cae/third_party/closure_compiler/compiled_resources2.gyp

FYI, attempted to start adding individual compile targets for each file at https://codereview.chromium.org/2892623002, but started hitting circular dependencies unfortunately (viewport -> zoom_manager -> viewport).

Wondering if a better approach is to just create a single target for main.js and have a flat list of all transitive dependencies (I think this would work around the circular deps, without having to restructure the code). Will try that too.

Patch2 at https://codereview.chromium.org/2892623002 shows the alternate approach of using a flat list. No circular deps problem anymore, down to 79 errors and 3 warnings!

I'll give it a shot to fix the most obvious errors first. Might need to delegate to someone more knowledgeable about the PDF codebase for more obscure errors.

Status update:
 1) Some errors being fixed at https://codereview.chromium.org/2892623002, no compile targets added yet.
 2) Some compile targets being added at https://codereview.chromium.org/2886943006.
 3) Adding externs for chrome.mimeHandlerPrivate API at https://codereview.chromium.org/2896433002 (necessary to compile browser_api.js).
 4) w3c_pointerlock.js externs included in Closure compiler's default externs at https://github.com/google/closure-compiler/issues/2502.
 5) chrome.tabs setZoom,getZoom,setZoomSettings,getZoomSettings externs being added to Closure compiler, WIP (necessary for browser_api.js).

After steps 4 and 5 a new Closure compiler version needs to be rolled to Chromium. Still have not addressed circular dependencies problems.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d2d647796babde3b2c5648e72b146d08d7e5c40d

commit d2d647796babde3b2c5648e72b146d08d7e5c40d
Author: dpapad <dpapad@chromium.org>
Date: Fri May 19 02:30:38 2017

Type check PDF plugin JS code, down to 53 errors.

BUG=721073
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2892623002
Cr-Commit-Position: refs/heads/master@{#473038}

[modify] https://crrev.com/d2d647796babde3b2c5648e72b146d08d7e5c40d/chrome/browser/resources/pdf/navigator.js
[modify] https://crrev.com/d2d647796babde3b2c5648e72b146d08d7e5c40d/chrome/browser/resources/pdf/open_pdf_params_parser.js
[modify] https://crrev.com/d2d647796babde3b2c5648e72b146d08d7e5c40d/chrome/browser/resources/pdf/toolbar_manager.js
[modify] https://crrev.com/d2d647796babde3b2c5648e72b146d08d7e5c40d/chrome/browser/resources/pdf/viewport.js
[modify] https://crrev.com/d2d647796babde3b2c5648e72b146d08d7e5c40d/chrome/browser/resources/pdf/viewport_scroller.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/3856b1d2931d37cef28514af93c291fbfc4d4eb2

commit 3856b1d2931d37cef28514af93c291fbfc4d4eb2
Author: dpapad <dpapad@chromium.org>
Date: Fri May 19 17:58:41 2017

WebUI: Add externs from chrome.mimeHandlerPrivate API.

BUG=721073
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2896433002
Cr-Commit-Position: refs/heads/master@{#473243}

[modify] https://crrev.com/3856b1d2931d37cef28514af93c291fbfc4d4eb2/extensions/common/api/PRESUBMIT.py
[modify] https://crrev.com/3856b1d2931d37cef28514af93c291fbfc4d4eb2/third_party/closure_compiler/externs/compiled_resources2.gyp
[add] https://crrev.com/3856b1d2931d37cef28514af93c291fbfc4d4eb2/third_party/closure_compiler/externs/mime_handler_private.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d8ec752daeb8bd6815a87a6d41e40c8a98f257d4

commit d8ec752daeb8bd6815a87a6d41e40c8a98f257d4
Author: dpapad <dpapad@chromium.org>
Date: Tue May 23 20:14:22 2017

PDF Plugin: Add compile targets for a few files.

 - gesture_detector
 - open_pdf_params_parser
 - pdf_scripting_api
 - viewport_scroller

BUG=721073
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2886943006
Cr-Commit-Position: refs/heads/master@{#474025}

[modify] https://crrev.com/d8ec752daeb8bd6815a87a6d41e40c8a98f257d4/chrome/browser/resources/pdf/compiled_resources2.gyp
[modify] https://crrev.com/d8ec752daeb8bd6815a87a6d41e40c8a98f257d4/chrome/browser/resources/pdf/gesture_detector.js
[modify] https://crrev.com/d8ec752daeb8bd6815a87a6d41e40c8a98f257d4/chrome/browser/resources/pdf/open_pdf_params_parser.js
[modify] https://crrev.com/d8ec752daeb8bd6815a87a6d41e40c8a98f257d4/third_party/closure_compiler/compiled_resources2.gyp

FYI remaining progress is blocked on https://codereview.chromium.org/2905553002, which rolls a new version of Closure compiler, with update externs definitions.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/062b17c082a63b24de989a5f79c77d6382e1aa15

commit 062b17c082a63b24de989a5f79c77d6382e1aa15
Author: dpapad <dpapad@chromium.org>
Date: Wed May 31 23:43:50 2017

Roll closure compiler

Change log:
https://github.com/google/closure-compiler/compare/ee44867683988d37c9cdb1b5c9557fe984c87135...e061fc7bd4ac37730534828a9484d528fcd80eb5
chrome_extensions.js: f9877b62e50844da51a50374dbc3c98d31d85ce3 -> 927e44d6e32d7b2f50ddb2b20f6515b79a0914b3
polymer-1.0.js: 5b180dcd7e8913c66386a675086b99aaad2f0a53 -> cc53ce238def6597f71ddd75e961011edb18e6a3

R=dbeam@chromium.org
BUG=721073
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2918683002 .
Cr-Commit-Position: refs/heads/master@{#476093}

[modify] https://crrev.com/062b17c082a63b24de989a5f79c77d6382e1aa15/chrome/browser/resources/md_bookmarks/list.js
[modify] https://crrev.com/062b17c082a63b24de989a5f79c77d6382e1aa15/chrome/browser/resources/md_bookmarks/types.js
[modify] https://crrev.com/062b17c082a63b24de989a5f79c77d6382e1aa15/third_party/closure_compiler/README.chromium
[modify] https://crrev.com/062b17c082a63b24de989a5f79c77d6382e1aa15/third_party/closure_compiler/compiler/compiler.jar
[modify] https://crrev.com/062b17c082a63b24de989a5f79c77d6382e1aa15/third_party/closure_compiler/externs/chrome_extensions.js
[modify] https://crrev.com/062b17c082a63b24de989a5f79c77d6382e1aa15/third_party/closure_compiler/externs/polymer-1.0.js

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e

commit 9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e
Author: dpapad <dpapad@chromium.org>
Date: Sat Jun 03 03:05:57 2017

PDF Plugin: Add compile targets for a few more files.

 - browser_api
 - viewer-bookmark
 - viewer-error-screen
 - viewer-page-indicator
 - viewer-page-selector
 - viewer-password-screen

BUG=721073
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2915773003
Cr-Commit-Position: refs/heads/master@{#476879}

[modify] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/browser_api.js
[modify] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/compiled_resources2.gyp
[add] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/elements/viewer-bookmark/compiled_resources2.gyp
[add] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/elements/viewer-error-screen/compiled_resources2.gyp
[modify] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/elements/viewer-error-screen/viewer-error-screen.js
[add] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/elements/viewer-page-indicator/compiled_resources2.gyp
[modify] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/elements/viewer-page-indicator/viewer-page-indicator.js
[add] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/elements/viewer-page-selector/compiled_resources2.gyp
[modify] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/elements/viewer-page-selector/viewer-page-selector.js
[add] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/elements/viewer-password-screen/compiled_resources2.gyp
[modify] https://crrev.com/9b2a8220cc2d621aad2a53f704b9ba9cd0ef183e/chrome/browser/resources/pdf/elements/viewer-password-screen/viewer-password-screen.js

I am not actively working no this, so marking as Available so others can continue. The basic setup is in place and some files are already being type checked.

per #3 sending over to thestig, since it sounds like compiler is setup.

Someone somewhere is laughing because they know how weak my JS-fu is.

I'll add it to my plate and see where it goes.

Setting PDF bugs assigned to me back to untriaged so they can get re-assigned as needed.